This invention relates to an apparatus and method for removing volatile and semi-volatile contaminants from solid materials by thermal desorption and, more particularly, to a bench scale apparatus and method for evaluating the effective interaction between soils and contaminants in order to determine the operating parameter requirements and applicability of thermal desorption for removal of organics and metals from soils.
The contamination of soils, sludges, ashes, and other solids by organics and metals is a significant environmental problem. Due to the large volumes involved and the expensive disposal costs for these solids, there is a need to reduce the volume of waste requiring disposal.
The contaminated soil may be treated by separation of the contaminants from the soil, such as by thermal desorption. During thermal desorption, the contaminants are heated under oxygen concentrations and residence time to a temperature effective to avoid decomposition of the contaminants, thereby enabling the separation of the volatilized contaminants from the soil.
As examples of thermal treatment of soil, the contaminated soil may be conveyed through a chamber by a screw conveyor with internally heated flights, placed within a rotary kiln, placed within a rotating cylinder having plural flights, or conveyed through a chamber on a conveyor belt.
An example of a thermal desorption unit which utilizes a conveyor belt is disclosed in U.S. Pat. No. 5,230,167 (Lahoda et al.), which is herein incorporated by reference. The method for removing contaminants from contaminated soil includes moving the contaminated material positioned on a belt conveyor through a chamber from an inlet end of the chamber to a discharge end of the chamber under oxygen concentration conditions, temperature conditions, and residence times effective to avoid incineration, combustion or oxidation of the contaminated material and the contaminants.
The contaminated material positioned on the belt conveyor in the chamber is heated, such as by a plurality of infrared heaters, either electric or gas fired, microwave, or other methods, positioned within or near the chamber, as the contaminated material moves through the chamber, to a temperature effective for volatilizing the contaminants and for producing a processed material that is substantially decontaminated.
A transport gas, such as steam, nitrogen, or carbon dioxide, is passed through the chamber, either cocurrent or countercurrent, to the movement of the contaminated material on the belt conveyor for transporting the volatile contaminants from the chamber. The volatilized contaminants are then discharged from the chamber to be further processed. The volatilized contaminants are condensed, separating the volatilized contaminants from the gas stream and producing a contaminated liquid condensate, which may be further processed.
The method may also include drying or wetting the contaminated material prior to entry of the contaminated material into the chamber, preventing air from entering the chamber, and scrubbing the condensed gas stream. To minimize the temperature gradient in the soil bed and to eliminate hot spots, the soil may be turned over several times using motorized rakes, chains or other means during the heating cycle.
To qualify the thermal desorption process, the applicability of thermal desorption to remove contaminants from the solids found at any particular site needs to be verified. To determine if the thermal desorption process is applicable to remove contaminants, the cost of operation, which requires the specification of operating parameters, such as residence time, power input, maximum required soil temperature, and effect of soil depth on the conveyor belt, needs to be determined.
Therefore, what is needed is a thermal desorption evaluation apparatus and procedure which may be performed on a sample of contaminated material for providing the operating parameters of a full scale process for particular contaminants and types of contaminated materials.